In addition to periodic incremental signals regarding the relative offset of two objects that are displaceable relative to one another, conventional position-measuring devices also supply so-called reference-pulse signals. With the aid of the reference-pulse signals, it is possible to obtain a precise absolute reference with regard to the position of the two mutually displaceable objects at individual specified reference positions along the measuring path. The incremental measurement, which typically has a much higher resolution, can be related to the absolute position determined in this manner. Numerous possibilities for generating the reference-pulse signals are convention.
In this context, reference is made, for example, to U.S. Pat. No. 5,073,710 and U.S. Pat. No. 5,981,941, both of which disclose variants of optical position-measuring devices. To generate the reference-pulse signals, diffractive reference-marking structures are mounted adjacent to the incremental track on the side of the measuring graduation at the reference position. Scanning of the reference marking produces a corresponding reference-pulse signal at the individual positions.
In particular in the case of high-resolution optical position-measuring devices operated in incident light, specific requirements will result with regard to the generation of the reference-pulse signals. For example, possible tilting of the scanning device and reflection-measuring graduation caused by, for example, a less than optimal installation of these devices should not cause any position error, if at all possible. This applies both to the generation of the incremental signals and the generation of the reference-pulse signals. The two above-referenced patent documents do not offer any suggestions for solving these problems.